OCR Text |
Show 66 STEPPES AND DESERTS. of the Harudsh, on the margin of the African Desert, reminds the geologist of the augitic vesicular amygdaloid, phonolite, and greenstone porphyry, which are only found at the northern and western boundaries of the Steppes of Venezuela and of the plains of the Arkansas, as it were on the hills of the ancient coast line. (Humboldt, Relation Historique, tom. ii. p. 142 i Long's Expedition to the Rocky Mountains, vol. ii. pp. 91 and 405.) (7) p. 26.-" When suddenly deserted by the east wind of the tropics in a sea covered with weed." It is a remarkable phenomenon, well known among sailors, that, in the vicinity of the African coast (between the Canaries and the Cape de Verde Islands, and particularly between Cape Bojador and the mouth of the Senegal), a west wind often takes the place of the general east or trade-wind of the tropics. It is the wide expanse of the Desert of Sahara which causes this westerly wind. The air over the heated sandy plain becomes rarefied, and ascends, the air from the sea rushes in to supply the void so formed, and thus there sometimes arises a west wind, adverse to sLips bound to the American coast, which are made in this manner to feel the vicinity of the heatradiating desert without even seeing the continent to which it belongs. The changes of land and sea breezes, which blow alternately at certain hours of the day or night on all coasts, are due to the same causes. The accumulation of sea-weed in the neighborhood of the African coast has been often spoken of by ancient writers. The locality of this accumulation is a problem which is intimately connected with our conjectures respecting the extent of Phrenician navigation. The Periplus, which has been ascribed to Scylax of Caryanda, and which, according to the researches of Niebuhr and Letronne, was very probably compiled in the time of Philip of Macedon, describes beyond Cerne a quantity of fucus forming a weed-covered sea-a kind of "Mar de Sargasso i" but the locality indicated appears to me to differ very much from that assigned in the work entitled "De 1\:Iirabilibus Auscultationibus, which long bore, unduly, the great name of Aristotle. (Compare Scyl. Caryand. Peripl. in Hudson, vol. ii. p. 53, with Aristot. de l\1irab. Auscult. in opp. omnia ex. |